Prior art printing presses employ several cylinders which cooperate with one another to produce printed copy. Specifically, a "plate" cylinder is used to mount the information to be printed. A "blanket" cylinder rotates about an axis parallel to the plate cylinder axis such that the surface of the blanket cylinder contacts the surface of the plate cylinder in a manner to transfer the image from the plate cylinder. The transferred image is the inverse of the image on the plate cylinder as is well understood.
A third cylinder, the "impression" cylinder, similarly rotates about a third axis parallel to the axis of the blanket cylinder in a manner so that the surfaces of the blanket and impression cylinders contact one another. The image on the impression cylinder is again inverted to the original image for imprinting the final image on a medium(a sheet of paper) moving between the blanket and impression cylinders.
Such an arrangement becomes much more complicated when more than one color is necessary for the final copy. Traditionally, a complete such system is necessary for each color needed. Moreover, a separate ink supply tower is necessary along with control means for ensuring that the different color images are properly superimposed on one another.
There are known processes in which multi-color prints can be made using a single plate cylinder. One such press arrangement is described in U.S. Pat. No. 3,233,541 issued Feb. 8, 1966 to O. D. Johnson. The Johnson press employs a single plate cylinder on the surface of which a plurality of plates are mounted. The plates are mounted in sequence so that a sequence of images is transferred to a blanket cylinder for retransfer to an impression cylinder. If the plates are inked with different color inks, a succession of (inverted) color images is formed on the surface of the blanket cylinder. Such images would be transferred in succession to an impression cylinder (actually to sheets of paper moving between the blanket and impression cylinders) but it is not obvious as to how such images would be superimposed to make a multi-color print.
Instead the Johnson press is used in a manner such that one color is applied to an entire image and a second color is applied on top of the first color area to only a portion of the entire image. The image formed by the second layer of ink is split off and deposited on the blanket cylinder with the underlying first layer of ink remaining on the plate cylinder. The Johnson apparatus employs "form" rollers which have different diameters over different positions of their lengths. In portions of a roller where the diameter is small, only the second color is transferred from the plate cylinder to the blanket cylinder; where large, transfer of the underlying color occurs.
The Johnson apparatus is not usable for forming a multicolor image where a set of different color images are superimposed on one another.
Another printing apparatus which is also sheet fed, employs two plate cylinders and a single blanket cylinder to obtain double colors. Such a system is available from Townsend Industries of Iowa.